1. Field of Invention
The present invention relates to a method for recovering reforming catalyst and more particularly to a method for classifying used catalysts with different degrees of aging, so reforming catalysts with a low degree of aging are collected easily to be recovered and reused.
2. Description of the Related Art
Catalytic reforming procedure is a main procedure in secondary processing of feed oil and is used to produce ingredients for gasoline, aromatic hydrocarbons and hydrogen cheaply in petroleum refineries. Catalytic reforming procedure categories include three kinds of reforming processes depending on processes for reforming catalysts, including a semi-regeneration reforming process, a continuous catalytic reforming process and a cyclic catalytic reforming process. In related industry, above processes are used in a proportion of 6:3:1. Now most new units adopt continuous catalytic reforming process.
The continuous catalytic reforming process comprises platinum (Pt)/tin (Sn) bimetallic catalysts having metallic properties and acidity. Furthermore, the continuous catalytic reforming process can be performed under an extremely low pressure (about 50 psig), which is useful for an aromatization reaction of oil and a conversion from gasoline with low octane value (such as straight-run gasoline, pyrolysis gasoline or the like) into gasoline with high octane value (such as motor gasoline, blending oils for aviation gasoline or the like) or petrified ingredients for refining benzene, toluene, xylene or the like.
Catalytic reforming procedure is performed under high temperature (490˜540° C.). After the catalytic reforming procedure, an activity of reforming catalysts is decreased due to carbon deposits and an increased agglomeration of Pt/Sn. In the continuous catalytic reforming process, the reforming catalysts can be activated by a carbon burning step, oxidation step, rejuvenation step, reduction step and chloriding step to reform and activate the used catalysts and maintain original activation of the used catalysts. However, total surface area of the used catalysts will be gradually decreased as the catalysts are used multiple times, so an operational life of the catalysts will be reduced.
With reference to FIG. 1, when a total surface area of reforming catalysts is decreased due to multiple use, a chloride content and a degree of metal dispersion in the reforming catalysts also decreases, which lowers the activation of reforming catalysts. For maintaining throughput of feed oil and quality of product, reaction temperature and added amount of dichloroethane should be increased, otherwise throughput of the feed oil should be lowered to maintaining product quality. Furthermore, phase form of aluminum oxide (Al2O3) support also affects the total surface area. Generally, a surface area of γ-Al2O3 is much larger than that of α-Al2O3, so the reaction temperature should be controlled to prevent γ-Al2O3 from converting to α-Al2O3.
Traditionally, the amount of the total surface area of the reforming catalysts is one of indexes for changing fresh catalysts. However, when a total output value of catalytic reformers decreases because properties of the catalysts are less preferential, a lost output value is more expensive than cost of the fresh catalysts. In other words, the catalysts are replaced by fresh catalysts at an economic point when costs of lost output outweigh replacement costs.
Under theoretical conditions, catalysts in a system have a same degree of aging to allow the catalysts to have the same activity and characteristics. But units have troubles sometimes, some catalysts may be more severely aging due to unusual operation conditions such as hot spot in certain area of catalyst cycle system. Under extraordinary conditions, catalysts are severely loss, so a large amount of fresh catalysts should be supplied. Under these unusual conditions, catalysts have different degrees of aging. If all catalysts are substituted, less aged catalysts are wasted. Economic benefits of the catalytic reformers will be affected if the catalysts are not substituted correctly.
Characteristics of reforming catalysts change greatly when the reforming catalysts are converted and deactivated from the γ-form Al2O3 support to α-form Al2O3 support, such as particle sizes of the reforming catalysts are decreased or a density of reforming catalysts is increased. Therefore, the reforming catalysts can be classified using screen or according to the density, as shown in U.S. Pat. No. 4,720,473. However, the particle sizes or the densities between reforming catalysts with different degrees aged do not present significant differences, so the reforming catalysts cannot be easily and effectively classified.
Currently, two methods for separating spent fluidized catalytic cracking catalyst include float/sink density separation (Beyerlein, R. A. et al., ACS Symposium Series 452, 109, 1990) and magnetic separation.
The more aged cracking catalyst, the more density of it. The float/sink density separation is usually used in laboratories and separates catalysts with different degrees of aging by adding used catalysts in a solution and adjusting density of the solution according to density of aged catalysts. Cracking catalyst consists of a certain amount of zeolite and a dimension of each molecule of the solution is chosen to be larger than a pore of zeolite, so molecules of the solution cannot enter into pores of the zeolite allowing the cracking catalysts to float on the solution. Therefore, the float/sink density separation is suitable for spent cracking catalyst. However, because reforming catalyst has large pores and most liquid solution is easily filled in the pores of the reforming catalyst and a solution with a density greater than the density of γ-form Al2O3 (3.97 g/cm3) is not easy obtained, especially without negative effect after separation.
U.S. Pat. No. 4,406,773 and U.S. Pat. No. 5,147,527 disclosed a separation of spent cracking catalyst by using magnetic field to recover spent cracking catalyst with low vanadium (V) content and low nickel (Ni) content. This method has been put into practice. However, less metal deposits on the reforming catalyst during reforming process and the reforming catalysts with different degrees of aging have the same content of metal. Therefore, this method cannot be used for separating reforming catalysts.
To overcome the shortcomings, the present invention provides a method for recovering reforming catalyst to mitigate or obviate the aforementioned.